1. Field of the Invention
The present invention relates to a conveyor oven using RF energy to heat a product as it moves through the oven.
2. Description of Related Art
Various types of conveyor ovens are used for cooking large volumes of products such as meat, fish or poultry, or for pasteurizing pre-packed products.
Conveyor ovens known in the art use a variety of heat sources to transfer heat to a product as it moves through the oven. For example, in a hot oil system, a pair of upper and lower platens are heated to a pre-selected temperature, typically in the range of 280° C., by means of recirculated oil that may be heated by an electric, steam or other source of heat energy. Separate conveyor belts are engaged by the upper pair of platens and the lower pair of platens, whereby the heat contained in the platens is transferred to the belts. Product is transported between the upper and lower platens and conveyor belts to heat the product. When the upper and lower platens are adjusted to be sufficiently close to each other they will effectively press a flat product such as bacon to prevent distortion or curling of the product. Alternatively, in a hot air system, heated air can be used to heat a “tunnel” formed between the upper and lower platens and belts to heat product as it passes through the formed tunnel. In an infrared heating system, an infrared energy source can be used to heat the product passing through the tunnel. Common to all of the aforementioned heating systems is the transfer of heat from the system to the outer surface of the product by convection. Internal heating of the product is by the conduction of heat through the product from its surface. Consequently, the thermal transfer properties of the product, and packaging, if the product is packaged prior to heating, are significant in determining the time-temperature heating profile for the product.
A well-known alternative to the above methods of convection heating is the direct application of microwave energy to the product within the range of approximately 915 MHz (typical commercial or industrial applications) and 2.4 GHz (typical for consumer or domestic applications). Lower frequencies, most notably 13.56 MHz and 27.12 MHz have been used for static (non-conveyor) baking and moisture leveling control, but not for high volume cooking of meat, fish, poultry and the like, or pasteurization of pre-packaged food products. Microwave energy excites molecules of water, fats, sugars and other components within a food product. The molecular excitation generates heat that is transferred to the product throughout its thickness. A magnetron is typically used as the energy source for the microwave energy. Magnetrons have a relatively short life in comparison with transistors or triodes used as energy sources in RF amplifiers. Since microwave energy generated by a magnetron is directed energy, placement of the magnetron relative to the product in the oven so as to ensure consistent neating throughout the product is problematic. There are also health and safety issues inherent with the use of high power microwave sources.
Pasteurization of food product, either pre-packaged or prior to packaging, involves keeping the food product throughout its volume at a pre-selected temperature, typically around 80° C. For a pre-packaged product, the product is passed through a water bath that is keep at the pre-selected temperature. As with the conduction heating methods described above, heat must be transferred by conduction from the water to the core of the product. Additionally some means of drying the packaging, such as a series of high pressure fans, must be utilized after the pasteurization process is complete. When the packaging is not a vacuum seal, air within the sealed packaging serves as a poor thermal conductor between the water bath and the product in the package.
Therefore, there exists the need for apparatus and a method of heating or pasteurization of a product that uses electromagnetic energy in frequency ranges below the microwave spectrum with a maximum power density and optimized operating frequency preferably within unlicensed Industrial, Scientific and Medicine (ISM) frequency bands.